We continue to develop mathematical models of human metabolism and body weight dynamics. Our mathematical model was recently used to provide the first quantification of feedback control of appetite in humans. We found that as people lose weight, appetite increases by about 100 kcal/d above baseline for every kilogram of lost weight. This feedback control mechanism helps explain why weight loss interventions typically result in a plateau within the first year and why it is so difficult to lose weight and keep it off over the long term.